For dry powder aerosols, dissolution into the lung medium is a critical factor in the fate of inhaled drugs. Slow dissolution can be used as a strategy to increase the duration of a drug in the lungs. However, slow dissolution may also cause a substantial fraction of inhaled particles to be redistributed to the gastro-intestinal tract with the mucociliary escalator or to be phagocytosed by alveolar macrophages and potentially degraded in their digestive organelles (Lundborg, Falk et al. 1992). Measuring dissolution of soluble particles in the lungs is very difficult. For more rapid-soluble substances indirect methods must be used such as the appearance of the solute in the circulation after an inhalation exposure. Slowly dissolved particles, such as some particulate radionuclides, are quantitated after the recovery of the particles from harvested lung tissues. A number of methods have been devised to simulate the rate of dissolution of solid particles in the lungs. Most of these methods are intended for complex particles of environmental origin with a low-solubility to insoluble core. Examples are radioactive particles (Kanapilly, Raabe et al. 1973), mineral particles or fibers (Johnson 1994). Recently, however, a filter method has been devised to simulate the rate of dissolution of drugs in the lungs (Davies and Feddah 2003). The drug particles are deposited on a glass fiber filter and then placed in a filter cassette. The filter is eluted in a flow-through configuration and the single pass eluate is repeatedly analyzed for the dissolved drug. There is room for improvements in finding more accurate, reliable and physiologically correct models to study how drug particles interact and dissolve in lung tissues.